<div class = "all">
  <div> 
    <button class="useless1"></button>
    <!--此按钮无用只是为了让背景图片铺满整个屏幕-->
</div>
  <div class="home"> 
    <button (click)="onClickToAnswerQuestion('科技')" >科技</button>
    <button (click)="onClickToAnswerQuestion('外交')" >外交</button>
    <button (click)="onClickToAnswerQuestion('经济')" >经济</button>
    <button (click)="onClickToAnswerQuestion('文娱')" >文娱</button>
  </div>

  <div> 
      <button class="useless2"></button>
      <!--此按钮无用只是为了让背景图片铺满整个屏幕-->
  </div>
</div>

